Plastic film having a high voltage breakdown

ABSTRACT

A stretched film includes a dispersion of at least one polyester and/or polycarbonate in a matrix of at least one polyester and/or polycarbonate different from the first polyester and/or polycarbonate, the percentage by weight of the dispersed polyester and/or polycarbonate in the dispersion being less than 50% and the dispersed polyester and/or polycarbonate being in the form of platelets. The stretched film can be used as a dielectric in a capacitor.

This application is a continuation of U.S. patent application Ser. No.14/324,641, filed 7 Jul. 2014, which is a continuation of U.S. patentapplication Ser. No. 12/937,143, filed 19 May 2011, which is theNational Phase filing of PCT Application No. IB2009/005217, filed 10Apr. 2009, which claims priority of European Application No. EP08290360.0, filed 11 Apr. 2008, the entireties of which applications areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a plastic film which can be used as adielectric film in capacitors. The invention also relates to the processof manufacturing such plastic film.

BACKGROUND OF THE INVENTION

A capacitor generally consists of two conducting metal plates separatedby an insulating media (dielectric film) capable of storing electricalenergy. The capacitor is formed by sandwiching the dielectric filmbetween the two conducting metal plates. The film serves as an insulantwhich prevents the electrons from moving from one capacitor plate to theopposite plate. The maximum energy that can be safely stored in aparticular capacitor is limited by the maximum electric field that thedielectric can withstand before it breaks.

The energy which can be stored by the film is proportional to thedielectric constant of the film and to the electrical breakdown of thedielectric as shown by the following equation.Energy density E=0.5·∈·∈₀·BDV²,

-   -   where BDV is the breakdown voltage (in V/μm),    -   ∈ is the theoretical relative permittivity of the film, also        named dielectric constant,    -   ∈₀ is the absolute permittivity.

The energy density of a capacitor containing a plastic film can beincreased by increase of the relative permittivity. This is obtained byuse of a polar polymeric material or by addition of a polar componentsuch as polar polymers or additives such as ceramic particles in thepolymeric film.

Capacitors in which a plastic film is used as the dielectric film withhigh relative permittivity are known in the art. For example, EP-A-0 039214 describes capacitors in which the dielectric film comprises avinylidene fluoride polymer, polycarbonate and/or a thermoplasticpolyester.

U.S. Pat. No. 6,426,861 describes a plastic film having a high energydensity. This film is made of a homogeneous mixture comprising at leastone non-polar homopolymer and at least one polar or non-polarhomopolymer. Examples of non-polar homopolymers are: polypropylene (PP),polyethylene naphthalate (PEN), polycarbonate (PC) and polyphenylenesulphide (PPS). An example of a polar polymer is polyvinylidene fluoride(PVDF). The homopolymers are blended and co-extruded with homogenizationto form a melt-cast hybrid copolymer dielectric film. This documentdescribes polymeric hybrid copolymers of PVDF and PP. It also describesa copolymer comprising PVDF and PP as one component and either PPS or PCor PET or PEN as a second component. The film disclosed in U.S. Pat. No.6,426,861 is prepared from homogeneous solid-solution mixtures.

The energy density of a capacitor containing a plastic film can beincreased by increase of the breakdown voltage of the plastic film.

EP-A-1 712 592 discloses a biaxially oriented film suitable as a filmcapacitor. This biaxially oriented film comprises a aromatic polyester(a) and a polyolefin (b) having a melting point of from 230 to 280° C.,wherein the ratio of the polyolefin (b) is from 2 to 60% based on theentire weight of the film. The polyolefin is preferably a styrenepolymer having a syndiotactic structure. It is preferred that the filmdoes not have voids. The voids formed at the boundary between thearomatic polyester (a) forming the matrix phase and the polyolefin (b)forming the island phase. In case where voids are present, the filmtends to be cut in the film stretching step. Further, as the filmthickness is reduced, the portions of the voids result in defects whichdeteriorate the mechanical characteristic or deteriorate the withstandvoltage characteristic. The voids can be eliminated by using acompatibilizing agent.

The present invention is directed to a plastic film having a highbreakdown voltage. This film can be used as a dielectric film in acapacitor. It imparts a high energy density to the capacitor.

The present invention is also directed to a process for increasing thebreakdown voltage of a plastic film by modification of the plastic filmmorphology.

SUMMARY OF THE INVENTION

The invention is directed to a stretched film comprising a dispersion ofat least one polyester and/or polycarbonate in a matrix of at least onepolyester and/or polycarbonate different from the first polyester and/orpolycarbonate, the percentage by weight of the dispersed polyesterand/or polycarbonate in the dispersion being less than 50% and thedispersed polyester and/or polycarbonate being in the form of platelets.

In one embodiment, the stretched film has a thickness in the range of0.3 to 25 μm, preferably in the range of 0.9 to 6 μm, more preferably inthe range of 2 to 4 μm.

In one embodiment, the largest dimension of said platelets is less than10 microns, preferably less than 5 microns, more preferably less than 1micron.

The invention is also directed to a stretched film comprising a mixturecomprising:

-   -   at least 50% by weight of a matrix of at least one polyester        and/or polycarbonate,    -   at least one polyester and/or polycarbonate miscible and        compatible with the at least one polyester and/or polycarbonate        constituting the matrix and forming a separate phase in the        matrix, the mixture being devoid of compatibilizer.

In one embodiment, in the said matrix of at least one polyester and/orpolycarbonate, the polyester is selected from the group comprisingpolyethylene terephthalate (PET), polyethylene naphthalate (PEN),polyethylene isophthalate, polybutylene terephthalate (PBT),polybutylene isophthalate, polybutylene naphthalate, polytrimethyleneterephthalate (PTT), polytrimethylene isophthalate, polytrimethylenenaphthalate, poly(cyclohexylene-dimethanol-terephthalate (PCT),polymethylene 1,3-propylene terephthalate, polyhexamethyleneterephthalate, polyisosorbide terephthalate (PEIT), polyhexamethylenenaphthalate, polyarylates (Par), copolymers thereof, mixtures thereofand liquid crystalline polyesters.

In one embodiment, in the said matrix of at least one polyester and/orpolycarbonate, the polycarbonate is selected from the group comprisingpolypropylene carbonate (PPC), polyphthalate carbonate, diphenylpolycarbonate (DPC), polyethylene terephthalate carbonate, polyethylenecarbonate, copolymers thereof and mixtures thereof.

In one embodiment, the at least one dispersed polyester or the at leastone miscible and compatible polyester is selected from the groupcomprising, polyethylene terephthalate (PET), polyethylene naphthalate(PEN), polyethylene isophthalate, polybutylene terephthalate (PBT),polybutylene isophthalate, polybutylene naphthalate, polytrimethyleneterephthalate (PTT), polytrimethylene isophthalate, polytrimethylenenaphthalate, poly(cyclohexylene-dimethanol-terephthalate (PCT),polymethylene 1,3-propylene terephthalate, polyhexamethyleneterephthalate, polyisosorbide terephthalate (PEIT), polyhexamethylenenaphthalate, polyarylates (Par), copolymers thereof, mixtures thereof,and liquid crystalline polyesters.

In one embodiment, the at least one dispersed polycarbonate or the atleast one miscible and compatible polycarbonate is selected from thegroup comprising polypropylene carbonate (PPC), polyphthalate carbonate,diphenyl polycarbonate (DPC), polyethylene terephthalate carbonate,polyethylene carbonate, copolymers thereof and mixtures thereof.

In one embodiment, the at least one dispersed polyester and/orpolycarbonate or the at least one miscible and compatible polyesterand/or polycarbonate is selected from polymers having a relativepermittivity in the range from 1 to less than 6, preferably from 1 to 4,preferably from 2 to 4, more preferably from 2.5 to 3.5.

In one embodiment, the stretched film is bi-axially stretched.

In one embodiment, the stretched film has an electrical breakdown,measured by the mean of 5×5 cm area, greater than 370 V/μm, preferablyin the range of 440 to 550 V/μm.

In one embodiment, the stretched film has an energy density higher than2.0 J/cm³, preferably in the range of 2.5 to 3.5 J/cm³ and morepreferably in the range of 3.5 to 4.5 J/cm³.

In one embodiment, the stretched film contains up to 60000 parts permillion of filler particles.

In one embodiment, the stretched film is devoid of any filler.

In one embodiment, the stretched film has a mechanical modulus inmachine direction (MD) or transverse direction (TD) higher than 1000N/mm², preferably in the range of 2500 to 4000 N/mm².

In one embodiment, the stretched film has a shrinkage in both machinedirection (MD) and transverse direction (TD) of less than 5% at atemperature of 150° C. and less than 15% at a temperature of 200° C.

In one embodiment, the dispersion consists of at least two polymersselected from the group consisting of polyesters and polycarbonates; thedispersion forming platelets each of which being formed by the at leasttwo polymers and the matrix consists of a polyester selected from thegroup consisting of polyethylene terephthalate (PET) and polyethylenenaphthalate (PEN); wherein the glass transition temperature of each ofthe polymers in the dispersion is in the range of Tgm+10° C. to Tgm+40°C., wherein Tgm is the glass transition temperature of the polyester ofthe matrix. In one embodiment, the dispersion is a mixture of apolycarbonate and a polyester and the polyester of the matrix ispolyethylene terephthalate (PET). The polyester of the dispersion ispolyethylene terephthalate (PET) or polybutylene terephthalate (PBT).

In one embodiment, the dispersion or the separate phase consists of apolycyclohexane-dimethanol-terephthalate (PCT) copolymer and thepolyester of the matrix is polyethylene terephthalate (PET).

The films produced in these last two embodiments can be used as anelectrical insulator film in power capacitors operating at voltagessuperior to 200 V and in a temperature range superior to 120° C.

In one embodiment, the dispersion or the separate phase consists of apolycarbonate (PC) and the polyester of the matrix is polyethylenenaphthalate (PEN).

The invention is also directed to a capacitor comprising a dielectricfilm which is the stretched film as described above.

The invention is also directed to the use of the stretched as a barrierto oxygen.

The invention is also directed to the use of the stretched film as anelectrical insulator film.

The invention is also directed to a process for making a stretched film,comprising the steps of:

a) providing a mixture of miscible and compatible polyesters and/orpolycarbonates, the percentage by weight of at least one polymer in themixture being at least 50%,

b) forming a film such that the at least one dispersed polymer is in theform of nodules,

c) stretching the film such that the at least one dispersed polymer isin the form of platelets,

d) heat treating the film.

In one embodiment, the film is bi-axially stretched.

In one embodiment, the film is simultaneously stretched.

In one embodiment, the stretching ratio is 2 to 7×, preferably 3.5 to4.5×.

In one embodiment, step c) is carried out at a temperature in the rangeof Tg+5° C. to Tg+30° C. where Tg is the highest glass transitiontemperature of the polymers in the mixture.

In one embodiment, step d) is carried out at a temperature in the rangeof Tm-80° C. to Tm-10° C., preferably in the range of Tm-70° C. toTm-20° C., where Tm is the highest melting temperature of the polymersin the mixture.

In one embodiment, the melt viscosities of the polyesters composing themixture are in the range of 50 Pa·sec to 5000 Pa·sec at a temperature Tmwhere Tm is the highest melting temperature of the polymers in themixture.

The invention is also directed to the use of a stretched film comprisinga dispersion of at least one polymer in a matrix of at least onedifferent polymer, for increasing the breakdown voltage of a capacitorfilm, the percentage by weight of the dispersed polymer in thedispersion being less than 50%, the dispersed polymer being in the formof platelets and having a relative permittivity less than 6.

In one embodiment, the absolute value of the difference between therelative permittivity of each polymer composing the dispersed phase andthe mean relative permittivity of the matrix phase is less than 2,preferably less than 1. The relative permittivity of a matrix phasecomprising two or more polymers can be determined by the followingformula. This formula defines a “mean” relative permittivity of thematrix phase ∈_(m):1/∈_(m)=Σ_(i) w _(i)/∈_(mi)

-   -   where ∈_(m) is the “mean” relative permittivity of the matrix        phase,    -   w_(i) is the weight percentage of the i^(th) polymer in the        matrix phase    -   ∈_(mi) is the relative permittivity of the i^(th) polymer in the        matrix phase

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 represents an electron microscopy photography of an unstretchedmixture comprising 30% of polycarbonate PC as a dispersed phase in apolyethylene naphthalate PEN matrix. The magnification rate is 10000.

FIG. 2 represents an electron microscopy photography of an unstretchedmixture comprising 20% polycarbonate PC as a dispersed phase in a matrixcomposed of PEN and PET. The magnification rate is 10000.

FIG. 3 represents an electron microscopy photography of a stretched film3.5×3.5 comprising 30% of polycarbonate PC as a dispersed phase in apolyethylene naphthalate PEN matrix. No voids are observed at theinterface between the dispersed phase and the matrix in the stretchedfilm. The magnification rate is 5000.

FIG. 4 represents an electron microscopy photography of a stretched film3.5×3.5 comprising 20% polycarbonate PC as a dispersed phase in a matrixcomposed of PEN and PET. No voids are observed at the interface betweenthe dispersed phase and the matrix in the stretched film. Themagnification rate is 5000.

FIG. 5 represents the breakdown voltage (expressed in V/μm) as afunction of the weight percentage of the dispersed phase for thedifferent films presented in the experimental section. The label on thedata points shown in FIG. 5 indicates the number of the example.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a stretched film obtained from apolymer mixture which is a dispersion of at least one polymer in amatrix of at least one different polymer. In the most preferredembodiment of the invention, the polymer mixture is a dispersion of atleast one first polyester and/or polycarbonate in a matrix of at leastone second polyester and/or polycarbonate different from the firstpolyester and/or polycarbonate. However, the person of ordinary skill inthe art will readily recognize how to apply the invention tocombinations of polymers other than polyesters and polycarbonates.

The polymer mixture is processed into a primary film which is thensubjected to stretching. The resulting film is characterized by a highbreak-down voltage, a high energy density and a high servicetemperature.

The at least one dispersed polymer forms a first phase distinct from thematrix which forms a second phase. The at least one dispersed polymerforms nodules (or islands) in the polymer matrix. In the presentinvention, the term nodule denotes a body having a substantially roundshape and which is characterized by a specific dimension ratio. Thisdimension ratio is the ratio of the nodule largest dimension to thenodule smallest dimension. It is less than 2, preferably less than 1.5.The smallest and the largest dimensions of the nodules are determinedthrough electronic microscopy, by first cutting the primary film acrossits section, not across its plane, and then analyzing the picture of theresulting section.

Electron microscopy can be used to show the phase separation and thepresence of nodules. FIG. 1 represents an electron microscopyphotography of a mixture comprising polycarbonate PC as a dispersedphase in a poly(ethylene naphthalate) PEN matrix. FIG. 2 represents anelectron microscopy photography of an unstretched mixture comprising 20%polycarbonate PC as a dispersed phase in a matrix composed of PEN andPET. The nodules are visible in the photography as rounded masses ofirregular shape. Their diameter is typically from 20 nm to 1000 nm.

The percentage by weight of the at least one dispersed polymer in themixture is less than 50% of the mixture weight.

The percentage by weight of the at least one dispersed polymer in themixture is preferably at least 5% of the mixture weight, more preferablyat least 10% of the mixture weight, and even more preferably at least25% of the mixture weight.

In one preferred embodiment, the phase separation is obtained byselecting polymers which are compatible with each other. Two polymersare said to be compatible when:

-   -   a) the polymers are miscible and it is possible to process them        into a two-phase system,    -   b) miscibility and copolymerization between the polymers occurs        only at the interface between the two phases,    -   c) miscibility and copolymerization at the interface prevents        the formation of voids between the two phases during operation        of primary film formation and subsequent stretching step.

Compatibility between the polymers means that the dispersed phaseadheres to the matrix phase. A high electrical breakdown voltagerequires good adhesion between the dispersed phase and the matrix phase,as well as no defects. Compatibility between the at least one polymer ofthe dispersed phase and the at least one polymer of the matrix phaseresults in a defect-free interface. There is no debonding between thedispersed phase and the matrix phase. The interface can be defined asthe boundary between the dispersed phase and the matrix phase. Theinterface width is typically from about 10 nm to 100 nm.

It is emphasized that because the polymers are compatible there is noneed to add a compatibilizer to the mixture phase. It is not necessaryto stabilize the interface between the dispersed phase and the matrixphase. The mixture which leads to the film of the invention is thuspreferably devoid of any compatibilizer.

In this preferred embodiment where only compatible polymers are presentin the mixture, said mixture is devoid of any polymer containing polargroups, i.e. the mixture is devoid of any polymer having a relativepermittivity higher than 6.

In one preferred embodiment, the mixture contains polyesters comprisinga carboxylic acid ester group R—CO—O—R′ wherein R and R′ denote anyalkyl or aryl group. The mixture also contains polycarbonates comprisinga carbonate group R—O—COO—R′ wherein R and R′ denote any alkyl or arylor amide or ether or aryl-ether or imides or ester group.

When the at least one polymer constituting the matrix phase is apolyester it can be selected from the group comprising polyethyleneterephthalate PET, polyethylene naphthalate PEN, polyethyleneisophthalate, polybutylene terephthalate PBT, polybutylene isophthalate,polybutylene naphthalate, polytrimethylene terephthalate PTT,polytrimethylene isophthalate, polytrimethylene naphthalate,poly(cyclohexane-dimethanol-terephthalate PCT, polymethylene1,3-propylene terephthalate, polyhexamethylene terephthalate,polyisosorbide terephthalate PEIT, polyhexamethylene naphthalate,polyarylates (Par), copolymers thereof and mixtures thereof and liquidcrystalline polyesters.

When the at least one polymer constituting the matrix phase is apolycarbonate it can be selected from the group comprising polypropylenecarbonate PPC, polyphthalate carbonate, diphenyl polycarbonate DPC,polyethylene terephthalate carbonate, polyethylene carbonate, copolymersthereof and mixtures thereof.

When the at least one polymer in the dispersed phase is a polyester itcan be selected from the group of comprising polyethylene terephthalatePET, polyethylene naphthalate PEN, polyethylene isophthalate,polybutylene terephthalate PBT, polybutylene isophthalate, polybutylenenaphthalate, polytrimethylene terephthalate PTT, polytrimethyleneisophthalate, polytrimethylene naphthalate,polycyclohexane-dimethanol-terephthalate PCT, polymethylene1,3-propylene terephthalate, polyhexamethylene terephthalate, polyisosorbide terephthalate (PEIT), polyhexamethylene naphthalate,polyarylates (Par), copolymers thereof and mixtures thereof, liquidcrystalline polyesters.

When the at least one polymer in the dispersed phase is a polycarbonateit can be selected from the group comprising polypropylene carbonatePPC, polyphthalate carbonate, diphenyl polycarbonate DPC, polyethyleneterephthalate carbonate, polyethylene carbonate, copolymers thereof andmixtures thereof.

Suitable types of polycarbonates can be selected from commercial brandssuch as MAKROLON® from Bayer, LEXAN® from General Electric and PANLITE®from Teijin and XANTAR® from DSM and IUPILON® from Mitsubishi andCALIBER® from Dow. Suitable types of polycarbonates mixtures can beselected from commercial blends such as MAKROBLEND® from Bayer, XENOY®from General Electric, VANDAR® from Ticona and SABRE® from Dow andSTAPRON® E from DSM and ULTRABLEND® from BASF. The present list isindicative and not exhaustive.

Suitable types of polyesters can be selected from commercial brands suchas TRISTAN®, EASTMAN®, EKTAR®, EASTAR®, KODAR® from Eastman andSKYGREEN® from SK Chemicals. Suitable types of liquid crystallinepolyesters can be selected from commercial brands such as VECTRA® fromTicona, Xydar® from Solvay or ZENITE® from DuPont. The present list isindicative and not exhaustive.

In one less preferred embodiment, the mixture contains incompatiblepolymers and a compatibilizer is added to the mixture to prevent theformation of voids between the two phases during operation of primaryfilm formation and subsequent stretching step.

In one embodiment, the mixture comprises polycarbonate as a dispersedphase in a matrix comprising polyethylene naphthalate and/orpolyethylene terephthalate. In one preferred embodiment, the mixturecomprises 5-15% polycarbonate PC as a dispersed phase and 85-95% ofpolyethylene naphthalate PEN as a matrix. In another preferredembodiment, the mixture comprises 25-35% polycarbonate PC as a dispersedphase and 65-75% of polyethylene naphthalate PEN and/or polyethyleneterephthalate PET as a matrix. In another preferred embodiment, themixture comprises 35-45% polycarbonate PC as a dispersed phase and55-65% of polyethylene naphthalate PEN as a matrix.

In one embodiment, the mixture comprisespolycyclohexane-dimethanol-terephthalate PCT or apolycyclohexane-dimethanol-terephthalate PCT copolymer as a dispersedphase in a matrix comprising polyethylene naphthalate PEN and/orpolyethylene terephthalate PET. In one preferred embodiment, the mixturecomprises 35-45% polycyclohexane-dimethanol-terephthalate PCT as adispersed phase and 55-65% of polyethylene naphthalate PEN and/orpolyethylene terephthalate PET as a matrix.

In one embodiment, the mixture comprises polycarbonate andpolycyclohexane-dimethanol-terephthalate PCT or apolycyclohexane-dimethanol-terephthalate PCT copolymer as a dispersedphase in a matrix comprising polyethylene naphthalate PEN and/orpolyethylene terephthalate PET.

In one embodiment, the mixture comprises polycarbonate PC and/orpolybutylene terephthalate PBT and/or polyethylene terephthalate PET andcopolymers thereof as a dispersed phase in a matrix comprisingpolyethylene naphthalate PEN and/or polyethylene terephthalate PETand/or polycarbonate PC and copolymers thereof.

In one preferred embodiment, the mixture comprises as a dispersed phase15-45% of a blend comprising either:

-   -   a) polyethylene terephthalate PET and polycarbonate PC, or    -   b) polybutylene terephthalate PBT and polycarbonate PC        and as a matrix 55-85% of polyethylene terephthalate PET. The        given percentages are weight percentages by weight of the        mixture.

In one embodiment, the mixture comprises polycarbonate PC andpolybutylene terephthalate PBT and copolymers thereof as a dispersedphase in a matrix comprising polyethylene naphthalate PEN and/orpolyethylene terephthalate PET and/or polycarbonate PC and copolymersthereof.

In one embodiment, the mixture comprises polycarbonate PC andpolyethylene terephthalate PET and copolymers thereof as a dispersedphase in a matrix comprising polyethylene naphthalate PEN and/orpolyethylene terephthalate PET and/or polycarbonate PC and copolymersthereof.

In one embodiment, the mixture comprises polyethylene terephthalate PETand polybutylene terephthalate PBT and copolymers thereof as a dispersedphase in a matrix comprising polyethylene naphthalate PEN and/orpolyethylene terephthalate PET and/or polycarbonate PC and copolymersthereof.

In one embodiment, the mixture comprises polyethylene terephthalate PETas a dispersed phase in a matrix comprisingpolycyclohexane-dimethanol-terephthalate PCT. In one preferredembodiment, the mixture comprises 35-45% polyethylene terephthalate PETas a dispersed phase and 55-65% ofpolycyclohexane-dimethanol-terephthalate PCT as a matrix.

In one embodiment, the mixture comprises polyethylene naphthalate PENand/or polycyclohexane-dimethanol-terephthalate PCT or apolycyclohexane-dimethanol-terephthalate PCT copolymer and/orpolytrimethylene terephthalate PTT and/or polytrimethylene isophthalateand/or polyethylene terephthalate PET or a polyethylene terephthalatePET copolymer and/or polybutylene terephthalate PBT or a polybutyleneterephthalate copolymer as a dispersed phase in a matrix comprisingpolycarbonate PC and/or a polycarbonate PC copolymer and/or polyethylenecarbonate.

In one embodiment, the mixture comprisespolycyclohexane-dimethanol-terephthalate PCT or apolycyclohexane-dimethanol-terephthalate PCT copolymer and/orpolybutylene terephthalate PBT or a polybutylene terephthalate copolymerand/or polytrimethylene terephthalate PTT or a polytrimethyleneterephthalate copolymer as a dispersed phase in a matrix comprisingpolycarbonate PC or polyethylene naphthalate PEN or polyethyleneterephthalate PET.

In one embodiment, the mixture comprises polybutylene terephthalate PBTas a dispersed phase in a matrix comprising polyethylene terephthalatePET or polycarbonate PC or polyethylene naphthalate PEN orpolycyclohexane-dimethanol-terephthalate PCT or apolycyclohexane-dimethanol-terephthalate PCT copolymer.

In one embodiment, the mixture can be composed of commercially availableblends such as MAKROBLEND® from Bayer, XENOY® from General Electric,VANDAR® from Ticona and SABRE® from Dow and STAPRON® E from DSM andULTRABLEND® from BASF. The present list is indicative and notexhaustive.

In one embodiment, the mixture can be composed of commercially availableblends such as TRISTAN®, EASTMAN®, EKTAR®, EASTAR®, KODAR® from Eastmanand SKYGREEN® from SK Chemicals. The present list is indicative and notexhaustive.

A mixture comprising a polyethylene terephthalate/polyethylenenaphthalate copolymer dispersed in a matrix comprising polycarbonate PCor polyethylene naphthalate PEN or polyethylene terephthalate PET canalso be considered.

In one preferred embodiment, the mixture comprises 15 to 45%polycarbonate PC as a dispersed phase in a matrix comprising 85 to 55%polyethylene naphthalate PEN.

In one preferred embodiment, the mixture comprises:

-   -   for the dispersed phase: at least two polymers selected from the        group consisting of polyesters and polycarbonates;    -   for the matrix: a polyester selected from the group consisting        of polyethylene terephthalate (PET) and polyethylene naphthalate        (PEN);    -   wherein the glass transition temperature of each of the polymers        in the dispersion is in the range of Tgm+10° C. to Tgm+40° C.,        wherein Tgm is the glass transition temperature of the polyester        in the matrix. The glass transition temperature for PET is about        75° C. The glass transition temperature for PEN is about 125° C.        A mixture comprising polyethylene terephthalate (PET) as a        matrix is particularly preferred.

It is advantageous that the dispersed phase comprises polymers having aglass transition temperature in the range of Tgm+10° C. to Tgm+40° C.,wherein Tgm is the glass transition temperature of the polyester in thematrix. This feature provides the following advantage: the platelets inthe stretched film exhibit an elliptic form which acts as brakes slowingdown the electrons motion and so reduce their kinetic energy.

A very good miscibility is in particular obtained when the dispersedphase comprises a mixture of a polycarbonate and a polyester; and thepolyester in the matrix is polyethylene terephthalate (PET) or apolybutylene terephthalate (PBT). The preferred polyester in thedispersed phase is polyethylene terephthalate (PET).

In one embodiment, the mixture is composed of commercially availableblends such as MAKROBLEND® from Bayer, XENOY® from General Electric,VANDAR® from Ticona and SABRE® from Dow and STAPRON® E from DSM andULTRABLEND® from BASF. The present list is indicative and notexhaustive.

As a result of the good miscibility and the elliptic form of theplatelets, a high breakdown voltage is obtained.

In a preferred embodiment, the mixture comprises a dispersed phaseconsisting of polycyclohexane-dimethanol-terephthalate (PCT) polymer orcopolymer and the matrix consists of polyethylene terephthalate (PET).

The mixture may also contain inorganic filler particles to improve thehandling and winding of the resulting film. Preferably the mixturecontains inorganic filler particles. Preferably the mixture containsfiller particles in an amount up to 60000 parts per million (ppm). Inone preferred embodiment, the stretched film contains up to 10000 partsper million of filler particles.

The inorganic filler particles may be without limitation calciumcarbonate, clays, silica, zeolites, silicone beads (functionalizedpolydimethyl siloxanes), dicalcium phosphates (DPC), tricalciumphosphates (TPC), cenospheres, zeeospheres, talc, titanium dioxides,barium sulfate and barium titanate. Filler particle size distributionscan be monomodal, bimodal and trimodal. Preferably the mean particlediameter is comprised between 0.1 and 10 micrometers for a monomodal,bimodal and trimodal distribution.

The different constituents of the mixture are mixed and the mixture isheated and stirred such as to enable the formation of a dispersed phasein the polymer matrix as explained hereafter. In the most preferredembodiment of the invention, the polymer mixture is a dispersion of atleast one first polyester and/or polycarbonate in a matrix of at leastone second polyester and/or polycarbonate different from the firstpolyester and/or polycarbonate.

It is important that the at least one polymer of the dispersed phasecopolymerizes with the at least one different polymer of the matrixphase only at the interface between the matrix phase and the dispersedphase. The process conditions must be selected such as to achieve ablend of compatible polymers. In the preferred embodiment, wherein thepolymer mixture comprises at least one polyester and/or polycarbonatedispersed in a matrix of at least one different polyester and/orpolycarbonate, it is emphasized that the film is not obtained throughprocess conditions which lead to copolymerization of the polyesters orpolycarbonates. Therefore, the process conditions such as temperatureand mixing time are to be selected such as to minimize the degree ofcopolymerization either between polyesters, or between polycarbonates orbetween a polyester and a carbonate. It is possible to achieve thefollowing structures:

-   -   a dispersed phase of at least one polyester in a matrix of at        least one different polyester, or    -   a dispersed phase of at least one polycarbonate in a matrix of        at least one different polycarbonate, or    -   a dispersed phase of at least one polyester in a matrix of at        least one polycarbonate, or    -   a dispersed phase of at least one polycarbonate in a matrix of        at least one polyester.

Compatibility between two polyesters is obtained by miscibility andester-ester exchange at the interface between the polyesters.

Compatibility between two polycarbonates is obtained by miscibility andcarbonate-carbonate exchange at the interface between thepolycarbonates.

Compatibility between a polyester and a polycarbonate is obtained bymiscibility and ester-carbonate exchange at the interface between thepolyester and a polycarbonate.

Compatibility between the dispersed phase and the matrix phase allowsbiaxial stretching especially at high loads of platelets.

The occurrence of a copolymerization reaction may be detected throughDifferential Scanning calorimetry (DSC) measurements.

The mixture initially contains at least two distinct polymers, which areeach characterized by a glass transition temperature. The presence ofthe at least two distinct polymers in the mixture can be confirmedthrough DSC measurements. Indeed, the DSC technique detects as manydistinct glass transition temperatures as the number of distinctpolymers present in the mixture.

During the mixing and heating step the amount of each polymer in themixture tends to decrease because of the occurrence of acopolymerization reaction between the polymers. If a copolymerizationreaction occurs between the polymers then the distinct glass transitiontemperatures disappear and a unique glass transition occurs describingthe formation of the copolymer. So assuming that copolymerization iscompleted, then the mixture contains only one copolymer and the DSCmeasurements indicate only one glass transition temperature peakcorresponding to the copolymer formed.

In order to avoid copolymerization between the polyesters it isdesirable to set a short mixing time. The mixing time is typically fromabout 5 minutes to about 40 minutes.

As an example of suitable temperature conditions, the mixture may beheated at a temperature T that is at least 5° C. greater than thehighest melting point of each polymer composing the mixture for a timesuch that the mixture still exhibits separate transition temperatures.

The temperature and the mixing time are preferably selected such thatthe mixture still exhibits separate glass transition temperatures. Inthe preferred embodiment, wherein the at least two distinct polymers arechosen from the group comprising polyesters and polycarbonates, theseprocess conditions lead to a blend of polyesters and/or polycarbonatesand not to a copolyester.

Melt viscosities of polymers composing the blends are preferably in arange of 50 Pa·sec to 5000 Pa·sec at a temperature Tm which is thehighest melting temperature of the polymers in the mixture.

The polymer mixture is then extruded through an extruder and a meltsystem. A primary film is formed through a slit die and cooled down on aquenching drum.

The primary film is then processed into a film. The stretching stepcauses the dispersed phase to change shape. The shape of the polyesternodules or islands changes from an initially substantially round shapeto an elliptic shape also called platelet-like shape. The term plateletdenotes a body having a substantially elliptic shape and which ischaracterized by a specific dimension ratio. This dimension ratio is theratio of the platelets largest dimension to the platelets smallestdimension. It is at least 2, preferably at least 3, and most preferablyat least 10. The smallest and largest dimensions are determined throughelectron microscopy by first cutting the film through its section, notthrough its plane, and then analyzing the picture of the resultingsection. Preferably, the largest dimension of the platelets is less than10 microns, preferably less than 5 microns, more preferably less than 1micron.

FIG. 3 represents an electron microscopy photography of a stretched filmcomprising PC as a dispersed phase in a PEN matrix. FIG. 4 represents anelectron microscopy photography of a stretched film comprising 20%polycarbonate PC as a dispersed phase in a matrix composed of PEN andPET. No voids are observed in both FIG. 3 and FIG. 4 at the interfacebetween the dispersed phase and the matrix in the stretched film.

FIG. 5 represents the breakdown voltage (expressed in V/μm) as afunction of the weight percentage of the dispersed phase in the polymermixture. Increasing the weight percentage of the dispersed phaseincreases the breakdown voltage. The fact that electrons are forced tomove around the platelet is responsible for the high breakdown voltageof the stretched film. It can also be seen from FIG. 5 that highbreakdown voltage values, i.e. higher than 400 V/micron, can beobtained. The invention is thus based on the finding that the presenceof platelets imparts a higher dielectric breakdown voltage to the film,which results in a film having a higher energy density. FIG. 5 showsthat the higher the percentage of platelets in the matrix the higher thebreakdown voltage. On the contrary, FIG. 5 shows on Example 14 that afilm prepared from a mixture of polymers which have copolymerized has ahomogenous structure. This film shows a low break-down voltage.

The platelet-like shape of the dispersed phase forms a barrier to theelectrons and the presence of a platelet-like dispersed polymer phaseprevents the electrons from crossing the width of the plastic film. Thepresence of the platelets also imparts a slower motion to the electrons.Platelets thus act as brakes slowing down the electrons motion and soreduce their kinetic energy. The electrons are forced to move around theplatelets by repulsion forces created by the charges located at theinterface of the dispersed platelets. The travel length of the electronsincreases when the percentage of platelets increases as indicated inFIG. 5. Further, the higher the ratio of the platelets largestdimensions to the platelets smallest dimension the higher the breakdownvoltage. The present invention is based on the discovery that thebreakdown voltage of a plastic film can be increased by slowing down theelectron motion across the film through the presence of platelets in thefilm. The presence of platelets creates a tortuosity which increases thetravel length of the electrons. Tortuosity can be defined as the ratioof the travel length of the electrons to the width of the plastic film.

In a preferred embodiment the morphology leading to a higher break-downvoltage can be achieved with a mixture of compatible polymers such aspolyesters and/or polycarbonates. The skilled person in the art canproduce a similar morphology with a mixture of incompatible polymers byaddition of compatibilizers.

The shape of the platelets can be varied according to the glasstransition temperature of the polymer constituting the dispersed phase.

The film is preferably bi-axially stretched. The primary film calledcast film may be bi-axially stretched in both directions transversedirection (TD) and machine direction (MD) by the mean of a sequential orsimultaneous process.

According to a preferred embodiment, the stretching ratio in eachdirection is in the range of 2 to 5 times, preferably from 3 to 4.5times.

When a simultaneous process is carried out, the film is stretched at atemperature in the range of Tg+5° C. to Tg+30° C. where Tg correspondsto the highest glass transition temperature of the polymers constitutingthe mixture. The obtained thickness is typically in the range of 0.5-20μm, preferably from 1.4 to 5 μm. The stretching temperature can beadapted in the case of a sequential stretch by the person skilled in theart.

The film then undergoes a heat treatment, which imparts dimensionalstability to the film. The film is heated at a high temperature in therange of Tm-80° C. to Tm-10° C., preferably in the range of Tm-70° C. toTm-20° C., where Tm is the highest melting temperature of the polymersin the mixture.

The film of the invention has a high service temperature. It can be usedbetween 100° C. and 130° C. It thus finds use in high temperature andhigh-energy storage applications like capacitor films in hybrid cars.

The film of the invention can advantageously be used as a barrier tooxygen.

The film of the invention can advantageously be used as an electricalinsulator film.

The following examples are illustrative of the invention and should notbe considered as limiting.

EXAMPLES

The following examples are illustrative of the invention and should notbe considered as limitative.

The properties measured on the film samples indicated shown in table Iare the following:

-   -   The thickness is measured by use of a digital point thickness        gage commercially available from Mahr Feinprüf (Militron) or        Heidenhain. The thickness of a 5 cm×5 cm square film is        determined by the average of 5 measuring points in the film        square. The thickness of the samples is given in microns in        Table I.    -   Breakdown voltage expressed in V per μm. The breakdown voltage        was measured on a 25 cm²-surface (5 cm×5 cm) on a film sample by        use of an aluminium foil and an electrode. A voltage ramp is        applied to the film by a commercially available power source (UN        Gerätebau). The breakdown voltage of the film is the maximum        voltage measured when a current superior to 5 mA flows through        the film sample. The breakdown voltage per thickness unit is the        maximum measured voltage divided by the film thickness. An        average of 5 measurements of breakdown voltage (expressed in        V/μm) is given in Table I.    -   Melting point expressed in ° C. as measured by differential        scanning calorimetry by mean of a commercially available        instrument from TA Instrument (model TA DSC 2920). The melting        point temperature is measured in a first heating scan.        Temperature scan rate is 20° C. per minute.    -   Glass transition point expressed in ° C. as measured by        differential scanning calorimetry by mean of a commercially        available instrument from TA Instrument (model TA DSC 2920). The        glass transition temperature is measured in a second heating        cycle after quenching. Temperature scan rate is 20° C. per        minute.    -   Mechanical modulus measured according to ASTM D-882-80        representing the stress at 1% elongation.    -   Dimensional stability measured by the thermal shrinkage of film        sample subject to a temperature of 150 and 200° C. during 30        minutes. The sample was a square piece of a film having a length        of 0.5 inch. The shrinkage is defined by the following formula:        ((initial length−length after shrinkage)/initial length)×100    -   Energy density expressed in J/cm³ indicating the energy the        dielectric film sample can store per volume unit. Energy density        is calculated by following equation:        Energy density E=0.5·∈·∈₀·BDV²,

where BDV is breakdown voltage (in V/μm),

-   -   ∈ is the theoretical relative permittivity of blends based on        their composition.

Theoretical permittivity is calculated by use of formula:1/∈=Σ_(i) w _(i)/∈_(i)

-   -   where ∈ is the relative permittivity of the film,        -   w_(i) is the weight percentage of the i^(th) component of            the film.        -   ∈_(i) is the relative permittivity of the i^(th) component            the dispersed phase at a temperature of 25° C. and a            frequency of 1 kHz.    -   ∈₀ is the absolute permittivity.

The breakdown voltage was first measured and the energy density wascalculated from the measured breakdown voltage.

All the following examples contain fillers particles having a size inthe range of 2-10 microns.

Comparative Example 1

A polyethylene naphthalate PEN polymer of an intrinsic viscosity of 0.63dl/g measured in 50% dichloroethane (DCE)/trifluoroacetic acid solutionis extruded in a twin-screw extruder. A cast film is obtained by formingthe polymer in a slot die system and by cooling on quench drum. The castfilm is subject to biaxial simultaneous stretching in laboratorystretcher (commercially available from companies Brueckner or T.M Longor Inventure laboratory). In this technology, the film is gripped byclips, which are simultaneously or sequentially moving in the machine ortransverse direction. The stretching is performed in a range oftemperature between 145 and 155° C. at a stretching ratio of 3.5 by 3.5.

The biaxially film obtained is between 6 and 15 μm thick. Table Iindicates the typical properties of the films.

Example 2

A polyethylene naphthalate PEN of an intrinsic viscosity of 0.63 dl/gmeasured in 50% dichloroethane (DCE)/trifluoroacetic acid solution ismixed with polycarbonate PC Makrolon® 2408 in an twin screw extruder inrespective amounts of 90% wt./10% wt. based on the total weight of thepolymeric mixture.

Stretching is similar to that in example 1, except that the stretchingtemperature is in the range of 150 to 170° C. and the stretching ratiois in range of 3.5 to 4×.

The biaxially stretched film obtained is between 6 and 15 μm thick andhas typical properties as indicated in table I.

Example 3

A polyethylene naphthalate PEN of an intrinsic viscosity of 0.63 dl/gmeasured in 50% dichloroethane (DCE)/trifluoroacetic acid solution ismixed with polycarbonate PC Makrolon® 2408 in an twin screw extruder inrespective amounts of 70% wt./30% wt. based on the total weight of thepolymeric mixture. Stretching is similar to that in example 2.

The biaxially stretched film obtained is between 6 and 15 μm thick andhas typical properties as indicated in table I.

Example 4

A polyethylene naphthalate PEN of an intrinsic viscosity of 0.63 dl/gmeasured in 50% dichloroethane (DCE)/trifluoroacetic acid solution ismixed with polycarbonate PC Makrolon® 3108 in an twin screw extruder inrespective amounts of 90% wt./10% wt. based on the total weight of thepolymeric mixture. Stretching is similar to that in example 2.

The biaxially stretched film obtained is between 6 and 15 μm thick andhas typical properties as indicated in table I.

Example 5

A polyethylene naphthalate PEN of an intrinsic viscosity of 0.63 dl/gmeasured in 50% dichloroethane (DCE)/trifluoroacetic acid solution ismixed with polycarbonate PC Makrolon® 3108 in an twin screw extruder inrespective amounts of 70% wt./30% wt. based on the total weight of thepolymeric mixture. Stretching is similar to that in example 2.

The biaxially stretched film obtained is between 6 and 15 μm thick andhas typical properties as indicated in table I.

Example 6

A polyethylene naphthalate PEN of an intrinsic viscosity of 0.63 dl/gmeasured in 50% dichloroethane (DCE)/trifluoroacetic acid solution ismixed with polycarbonate PC Makrolon® 3108 and a polycarbonate PC® 2408in an twin screw extruder in respective amounts of 70% wt./15% wt/15%wt. based on the total weight of the polymeric mixture. Stretching issimilar to that in example 2.

The biaxially stretched film obtained is between 6 and 15 μm thick andhas typical properties as indicated in table I.

Example 7

A polyethylene naphthalate PEN of an intrinsic viscosity of 0.63 dl/gmeasured in 50% dichloroethane (DCE)/trifluoroacetic acid solution ismixed with polycarbonate PC Makrolon® 2408 and a polyethyleneterephthalate PET in an twin screw extruder in respective amounts of 70%wt./20% wt./10% wt. based on the total weight of the polymeric mixture.Stretching is similar to that in example 2, except that stretchingtemperatures are in the range of 150 to 160° C.

The biaxially stretched film obtained is between 6 and 15 μm thick andhas typical properties as indicated in table I.

Example 8

A polyethylene naphthalate PEN of an intrinsic viscosity of 0.63 dl/gmeasured in 50% dichloroethane (DCE)/trifluoroacetic acid solution ismixed with polycarbonate PC Makrolon® 3108 in an twin screw extruder inrespective amounts of 65% wt./35% wt. based on the total weight of thepolymeric mixture. Stretching is similar to that in example 2, exceptthat stretching temperatures are in the range of 150 to 160° C.

The biaxially stretched film obtained is between 6 and 15 μm thick andhas typical properties as indicated in table I.

Example 9

A polyethylene naphthalate PEN of an intrinsic viscosity of 0.63 dl/gmeasured in 50% dichloroethane (DCE)/trifluoroacetic acid solution ismixed with polycarbonate PC Makrolon® 3108 in an twin screw extruder inrespective amounts of 60% wt./40% wt. based on the total weight of thepolymeric mixture. Stretching is similar to that in example 2, exceptthat stretching temperatures are in the range of 150 to 160° C.

The biaxially stretched film obtained is between 6 and 15 μm thick andhas typical properties as indicated in table I.

Example 10

A polyethylene naphthalate PEN of an intrinsic viscosity of 0.63 dl/gmeasured in 50% dichloroethane (DCE)/trifluoroacetic acid solution ismixed with polycarbonate PC Makrolon® 3108 in an twin screw extruder inrespective amounts of 80% wt./20% wt. based on the total weight of thepolymeric mixture. Stretching is similar to that in example 2, exceptthat stretching temperatures are in the range of 150 to 160° C.

The biaxially stretched film obtained is between 6 and 15 μm thick andhas typical properties as indicated in table I.

Example 11

A polyethylene terephthalate PET of an intrinsic viscosity of 0.59 dl/gmeasured in 40% tri-chloro-ethane (TCE)/phenol solution is mixed with anamorphous blend with an single glass transition of 100.8° C. composed ofa polycarbonate PC and a polyester in an twin screw extruder inrespective amounts of 70% wt./30% wt. based on the total weight of thepolymeric mixture. Stretching is similar to that in example 2, exceptthat stretching temperatures are in the range of 105 to 115° C.

The biaxially stretched film obtained is between 6 and 15 μm thick andhas typical properties as indicated in table I.

Example 12

A polyethylene terephthalate PET of an intrinsic viscosity of 0.56 dl/gmeasured in 40% tri-chloro-ethane (TCE)/phenol solution is mixed with anpolycyclohexane-dimethanol-terephthalate PCT copolymer Eastman A150 ® inan twin screw extruder in respective amounts of 70% wt./30% wt. based onthe total weight of the polymeric mixture. Stretching is similar to thatin example 2, except that stretching temperatures are in the range of 95to 105° C.

The biaxially stretched film obtained is between 6 and 15 μm thick andhas typical properties as indicated in table I.

Both films of Examples 11 and 12 have shown to be particularly wellsuited for making power capacitors. Known power capacitors generallycontain a dielectric film made of polypropylene (PP). This type ofcapacitors cannot be operated at a temperature higher than 105° C. Acapacitor comprising the film of Example 11 or Example 12 according tothe invention can be operated at a temperature in the range of 105° C.to 125° C. It is thus more stable at high temperature than a capacitorcomprising a PP dielectric film. A capacitor comprising the film ofExample 11 or Example 12 can thus be used for replacing power capacitorscomprising PP films.

Comparative Example 13

A polyethylene naphthalate PEN of an intrinsic viscosity of 0.63 dl/gmeasured in 50% dichloroethane (DCE)/trifluoroacetic acid solution ismixed with a polyethylene terephthalate PET of an intrinsic viscosity of0.55 dl/g measured in 40% tri-chloro-ethane (TCE)/phenol solution in antwin screw extruder in respective amounts of 85% wt./15% wt. based onthe total weight of the polymeric mixture.

Stretching is similar to that in example 2, except that stretchingtemperatures are in the range of 140 to 150° C.

The biaxially stretched film obtained is between 4 and 6 μm thick andhas typical properties as indicated in table I.

The film of Comparative Example 13 comprises a single phase.

Comparative Example 14

A polymer mixture composed of 47.5% wt. of polybutylene terephthalate(PBT) and 52.5% wt. of an isophthalate copolyester is mixed in a twinscrew extruder. The mixture is processed to achieve a copolymerizationboth in the melt system and in the extruder. The cast film is subject tostretching. The typical stretching temperature is in the range of 70 to100° C. The cast film and the stretched film show a homogeneousstructure. The break-down voltage of a 4 micron thick film consisting ofthe above composition shows a break-down voltage of 356 V/micron and anenergy density of 1.85 J/cm³. This comparative example clearly showsthat a single copolyester shows a low breakdown voltage and a low energydensity.

In contrast, examples 2-12 which are according to the invention andwhich exhibit a phase dispersed in a matrix exhibit improved electricalproperties.

Table I hereafter summarizes the typical properties of the stretchedfilm prepared according to the above examples.

TABLE I Mod- Shrinkage Shrinkage relative thick- Energy melting ulus 150deg C. 200 deg C. Exam- Permit- ness BDV density Tg 1 Tg 2 point (N/ (%)(%) ple tivity (mu) (V/mu) (J/cm3) (deg C.) (deg C.) (deg C.) mm2) MD TDavg. MD TD avg. 1 3.05 6.4 368 1.83 118.5 — 265.8 5450 1.2 1.6 1.4 3.6 43.8 2 3.03 5.0 444 2.65 118.7 138.7 265.6 5281 1 1 1 4 4.4 4.2 3 3.007.6 474 2.99 118.7 137.5 266.5 4642 1.2 1.8 1.5 6 7 6.5 4 3.3 6.7 4532.75 117.1 139.4 267.3 5000 0.4 1 0.7 3.4 5 4.2 5 3.00 5.8 575 4.39119.9 141.8 267.1 4609 1.4 1.8 1.6 7.2 7.4 7.3 6 3.00 8.2 453 2.73 119.7140.3 266.6 4684 1.6 2 1.8 7.6 8 7.8 7 3.4 8.1 455 2.78 109.5 138.3259.5 4089 0.8 0.4 0.6 2 1.4 1.7 8 3.00 5.2 508 3.42 115.5 133.5 264.23313 0.8 0.8 0.8 2.2 2.4 2.3 9 2.99 6.5 543 3.90 116.9 132.9 265 34330.6 1 0.8 3.2 2.8 3 10 3.02 8.5 446 2.66 118.4 139.2 263.6 4878 1.4 1.51.5 5.1 6.5 5.8 11 3.27 6.3 448 2.90 74.7 101.1 252.3 3613 2.4 2 2.2 7 77 12 3.24 7.0 498 3.55 74.1 86.6 253.2 2820 1.6 1.6 1.6 5.6 5.2 5.4 133.09 6.0 367 1.84 110.9 — 254.2 5276 1 0.7 0.9 2.8 2.5 2.7 14 356 1.85

The films of examples 2-12, all in the thickness range of 4.5 to 8.5 μm,show more than one glass transition temperature and a structurecomprising platelets. These films exhibit a break-down voltage of atleast 440 V/μm.

In contrast, Comparative Examples 1, 13 and 14 show a homogeneousstructure and a single glass transition temperature. The films ofcomparative examples 1, 13 and 14 exhibit a break-down voltage of only368, 367 and 356 V/μm, respectively, thus lower than the breakdownvoltages of examples 2-12.

The films of examples 2-12 exhibit an energy density of at least 2.6J/cm³ whereas the films of comparative examples 1, 13 and 14 exhibit anenergy density of only 1.83, 1.84 and 1.85 J/cm³ respectively, a typicalvalue for a polyester film.

These examples confirm that the film of the invention can withstand ahigher breakdown voltage and has a higher energy density.

The invention claimed is:
 1. A process of preparing a capacitorcomprising the steps of: a) providing a mixture comprising a dispersionof at least one polymer in a matrix of at least one different polymer,the percentage by weight of the dispersed polymer in the dispersionbeing from 30 to less than 50%; b) extruding the mixture through anextruder to form a primary film, the dispersed polymer having asubstantially round shape in the primary film prior to stretching; c)stretching the primary film to form a biaxially stretched film, thedispersed polymer being in the form of platelets and having a relativepermittivity of less than 6, the ratio of the platelets largestdimension to the platelets smallest dimension being at least 3, whereinsubstantially no voids are present between the dispersed polymer and thematrix in the biaxially stretched film; and d) forming the capacitorcontaining a dielectric, the dielectric being the stretched film.
 2. Theprocess according to claim 1, wherein the absolute value of thedifference between the relative permittivity of each dispersed polymerand the mean relative permittivity of the matrix is less than
 2. 3. Theprocess according to claim 2, wherein the absolute value of thedifference between the relative permittivity of each dispersed polymerand the mean relative permittivity of the matrix is less than
 1. 4. Theprocess according to claim 1, wherein the dispersed polymer has arelative permittivity in the range of from 1 to less than
 6. 5. Theprocess according to claim 1, wherein the dispersed polymer has arelative permittivity in the range of from 2 to
 4. 6. The processaccording to claim 1, wherein the dispersed polymer has a relativepermittivity in the range of from 2.5 to 3.5.
 7. The process accordingto claim 1, wherein the ratio of the platelets largest dimension to theplatelets smallest dimension is at least
 10. 8. The process according toclaim 1, wherein the stretched film has a thickness in the range of 0.3to 15 μm.